1. Field of the Invention
The present invention relates generally to flight control systems, and particularly to fly-by-wire flight control systems for unmanned airborne vehicles (UAVs).
2. Technical Background
The market for UAVs is growing and is in the range of several billion dollars per year. UAVs may be used for many purposes including aerial surveillance, weapons delivery, and target training. Many UAVs are used as target drones by providing military pilots with realistic, high performance targets during airborne training. Irregardless of the use, one method for making a UAV is by converting a retired man-rated aircraft into an unmanned vehicle that is remote controlled or preprogrammed to follow a predetermined trajectory. The process of conversion typically involves modifying the retired aircraft's flight control system. A discussion of basic aircraft terminology may be useful before presenting some of the conventional approaches for converting retired aircraft into target drones.
Note that a typical aircraft includes a fuselage, wings, one or more engines, and a tail section that includes horizontal stabilizers and a vertical stabilizer. The engines generate the thrust that drives the aircraft forward and the wings provide the lift necessary for the aircraft to become airborne. Control surfaces are disposed on the wings, the horizontal stabilizers and the vertical stabilizer. The control surfaces enable the aircraft to respond to the flight control system command inputs provided by the pilot(s) by directing air flow in a controlled manner. The major control surfaces disposed on the typical aircraft are the ailerons, the elevators, and the rudder.
The ailerons are disposed on the trailing edges of the wings and are used to control the roll of the aircraft. Roll refers to the tendency of the aircraft to rotate about the aircraft's central longitudinal axis. If the pilot moves the control stick (or alternatively the control wheel) to the left, the left aileron will rise and the right aileron will fall and the aircraft will begin rolling to the port side. In like manner, if the control stick is moved to the right, the aircraft will roll to the starboard side. The elevators are disposed on the rear edges of the horizontal stabilizers and are used to control the aircraft pitch. Pitch refers to the tendency of the aircraft to rotate around the transverse axis of the aircraft. For example, if the pilot adjusts the control stick aft, the elevators will cause the nose to pitch upward and the aircraft will tend to lose airspeed. If the stick is moved foreword, the nose of the aircraft pitches downward.
The rudder is disposed on the vertical stabilizer and is usually employed to adjust the yaw of the aircraft. The yaw is the tendency of the aircraft to rotate around the vertical axis, i.e., the axis normal to the longitudinal axis and the transverse axis. The rudder is typically controlled by a pair of foot-operated pedals.
The aircraft may also include secondary control surfaces such as spoilers, flaps, and slats. The spoilers are also located on the wings and are employed for a variety of functions. The flaps and the slats are also disposed on the wing and are typically used to adjust the aircraft's lift and drag during landing and take off. As noted above, the means for transmitting the pilot's commands to the above described control surfaces is commonly referred to as the flight control system.
In the description provided above, the most common control surfaces were discussed. However, those of ordinary skill in the art will understand that aircraft may employ other such control surfaces such as flaperons, elevons, ruddervators, and thrust vectoring nozzles to name a few. A flaperon is a combination flap and aileron and is used, for example, on the F-16. An eleven is a combination elevator and aileron and is used on flying wing aircraft and delta-wing aircraft such as the B-2, F-106, B-58, etc. The ruddervator is a combination of the rudder and the elevator and is used, for example, on the F-117. The F-22 also employs a specialized control surface known as a thrust vectoring nozzle in addition to the horizontal stabilizer.
The flight control system is designed to actuate the control surfaces of the aircraft, allowing the pilot to fly the aircraft. The flight control system is, therefore, the control linkage disposed between the control input mechanisms, i.e., the control stick, pedals and the like, and the control surface actuator devices. One criteria of flight control system design relates to the aircraft's handling characteristics. The flight control system is also designed and implemented in accordance with certain specifications that ensure a very high level of reliability, redundancy and safety. These issues are especially important for man-rated aircraft, i.e., those that are to be flown by a pilot, and carry aircrew or passengers. The system's reliability and redundancy ensures that there is a very low probability of failure and the resulting loss of the aircraft and life due to a control system malfunction. All of these factors ensure that the airplane can be operated safety with a minimum risk to human life.
In older aircraft, the control stick and the pedals are coupled to the control surfaces by a direct mechanical linkage. The pilot's commands are mechanically or hydraulically transferred to the control surface. The pilot's control inputs are connected to hydraulic actuator systems that move the control surfaces by a system of cables and/or pushrods. In recent years, aircraft having flight control systems featuring direct mechanical linkages have been replaced by newer aircraft that are equipped with an electrical linkage system commonly referred to as a fly-by-wire system.
A fly-by-wire system translates the pilot's commands into electrical signals by transducers coupled to the control stick and the pedals. The electrical signals are interpreted by redundant flight control computers. Thus, the flight control system performs multiple digital or analog processes that combine the pilot's inputs with the measurements of the aircraft's movements (from its sensors) to determine how to direct the control surfaces. The commands are typically directed to redundant control surface actuators. The control surface actuators control the hydraulic systems that physically move the control surface of the aircraft.
After a man-rated aircraft is retired, it may be re-used for airborne missions that do not require a pilot or on-board crew. This type of aircraft, known as an Unmanned Air Vehicle (UAV) or Target Drone is modified to take advantage of the existing systems by replacing the functionality typically provided by a pilot. The flight control system may be changed in order to allow control by a ground controller. Alternatively, conversion is implemented by modifying flight control processor logic to merge external sensor signals and commands into the control surface commands that drive the UAV.
Currently, the primary aircraft employed for full-scale target missions is the F-4 Phantom fighter aircraft, which is a 1960's vintage aircraft. Retired F-4 Phantom aircraft have been used as target drones for several years. Approximately 5,000 F-4s were produced over the years. Unfortunately, the fleet of available F-4 aircraft is dwindling and the supply of F-4 aircraft will soon be depleted. This problem may be solved by pressing newer retired fly-by-wire aircraft (such as the F-16 or F-18) into service to meet the demand for target drones. However, it must be noted that the F-4 Phantom is not a fly-by-wire system. The F-4 is equipped with an older hydro-mechanical flight control system. Accordingly, different technological means are required to convert the newer fly-by-wire aircraft into target drones.
In one approach, fly-by-wire conversion methods requiring flight control computer re-programming are being considered. In another approach that is being considered, the flight control computer is removed altogether and replaced with a new computer. The new computer is programmed to perform the functions normally performed by the pilot, in addition to the traditional flight control system functions. However, both of these approaches have their drawbacks. Reprogramming or replacing the original man-rated flight control processor is a complex and costly proposition. The new flight control processor has to pass many, if not all, of the aircraft development tests originally required. The fact that most of the fly-by-wire aircraft expected to be used for this application are now more than 20 years old further complicates matters. The designers of the new replacement systems are faced with replicating the original system's functions and capabilities without having the necessary documentation. The system design and test definitions for these functions have been lost over time.
Accordingly, the effort required to replicate and prove a replacement system having identical fit/form/function and repeat the required development testing has been found to be prohibitively expensive. What is needed is an alternative, and less expensive, method for converting retired fly-by-wire aircraft into UAVs and/or target drones.